In place of the vertical type continuous casting process which comprises casting steel by vertically withdrawing a cast strand from a vertical mold installed below a tundish, the horizontal type continuous casting process which comprises casting steel by horizontally withdrawing a cast strand from a horizontal mold installed at the lower part of the side wall of a tundish is recently being industrialized because of the low installation costs and other advantages.
The construction of the junction between the tundish and the horizontal mold in a horizontal type condinuous casting machine used in the above-mentioned horizontal type continuous casting process is illustrated in the form of a schematic sectional view in FIG. 1. In FIG. 1, 1 is a tundish; 1a is a steel shell of the tundish 1; 1b is a molten steel outlet pierced through the steel shell 1a, to which fitted is a seating ring 6 to which fitted inside is a horizontal front nozzle 2 having a recess for engaging with a feed nozzle. Also in FIGS. 1, 3 is a feed nozzle horizontally connected to the top of the front nozzle 2, and a break ring 5 is fitted to the top of the feed nozzle 3; and 4 is a water-cooled type mold, connected horizontally, through the break ring 5, to the top of the feed nozzle 3. Molten steel contained in the tundish 1 is poured through the front nozzle 2, the feed nozzle 3, and the break ring 5, into the mold 4 to be cast into a cast strand, which is then horizontally withdrawn from the mold 4.
In the above-mentioned horizontal type continuous casting machine, the side of the break ring 5 in contact with molten steel is heated to a high temperature, whereas the side in contact with the water-cooled mold 4 remains at a low temperature. In the break ring 5 under such severe conditions, cracks and breakage may easily occur.
Therefore, a break ring should have properties satisfying the following conditions:
The generally known materials for a break ring include alumina, zirconia, molten silica, silicon nitride, boron nitride and zirconia/molybdenum cermet. A break ring made of alumina or zirconia, from among the above-mentioned materials, has a low thermal stress resistance and tends easily to produce cracks. A break ring made of molten silica has its portion, which is in contact with molten steel, melting vitreously. Therefore, a break ring made of any one of alumina, zirconia and molten silica cannot satisfy the above-mentioned conditions. On the other hand, a break ring made of any one of silicon nitride, boron nitride and zirconia/molybdenum cermet is believed to almost satisfy the above-mentioned conditions. Particularly, a break ring made of silicon nitride is considered to be optimum because of its relatively low cost.
The break ring made of silicon nitride is manufactured by machining a silicon nitride sinter. For manufacturing a silicon nitride sinter, the following methods are generally known: